The invention relates generally to electronic amplifiers, and deals more particularly with a multistage amplifier which operates at high speed and requires only a low power supply voltage.
For some applications, such as a fiber optic digital receiver, high gain and high speed/high band width are essential. For example, some fiber optic cables used for digital communication between computers are required to transmit and receive 800 megabits per second, and faster speeds will be required in the future. Also, it is preferable to operate the amplifier from the same power supply used for the digital signals, and such a power supply may provide a low voltage. For example, if the digital signals are at a TTL level, the power supply voltage is five volts.
Many types of amplifiers were previously known. For example, a very common amplifier comprises a bipolar transistor, a large DC biasing resistor connected between the base of the transistor and the power supply, an emitter resistor connected between the emitter of the transistor and ground, and a collector resistor connected between the collector of the transistor and the power supply. Such an amplifier is useful for moderate speeds. However, the transistor in the amplifier inherently includes a parasitic capacitance at the base, and this capacitor is located in parallel with the base resistor. Consequently, the amplifier has a substantial RC time constant at its input, which time constant reduces the speed and band width of the amplifier. Differential amplifiers were also known for increased sensitivity.
It was also known to use "Gilbert Cells" to provide a high speed/high band width amplifier. These cells can be combined to yield high gain. However, each cell requires 0.5-1.0 volts of power supply voltage swing to operate, and the cells are stacked "vertically" from the power supply to the ground to combine them. Consequently, the maximum gain possible with a small power supply voltage such as 5 volts is limited.
There is yet another requirement that the fiber optic amplifier should meet. The photo signals impinging at the input of the amplifier may vary by a factor of 1000 in energy due to the proximity of the LED or LASER to the corresponding photosensitive diode, fabrication tolerance and other factors, and the amplifier should compensate for these variations to provide a manageable voltage output range. In the prior art, single ended amplifier noted above having the base and other resistors, the transistor will saturate to limit the output voltage. However, the saturation condition slows the subsequent response of the transistor, i.e. turn-off. The Gilbert Cell also is not suitable for a wide range of inputs because vertical stacking limits the allowable DC component that an input can have in relation to the power supply voltage.
Accordingly, a general object of the present invention is to provide a high speed/high band width amplifier.
Another general object of the present invention is to provide a multistage amplifier of the foregoing type which can operate at a low power supply voltage.
Another object of the present invention is to provide an amplifier of the foregoing types which is operable over a wide range of signal inputs, and yields an output within a manageable voltage range.
Still another object of the present invention is to provide an amplifier of the foregoing types which can be fabricated as a densely populated integrated circuit.